This invention relates to industrial apparatus, namely a pulveriser or grinding mill, in which pieces of a material are pulverised into a finer particulate form. The invention relates particularly, but not exclusively, to a mill in which coal is pulverised into a powder form which is conveyed to combustion apparatus e.g. of a power station.
In particular the invention concerns a mill having a lower grinding ring, which may be a part formed with an annular depression. Grinding elements are sandwiched between the lower grinding ring and a top member, which may have an annular depression facing an annular depression in the grinding ring. The grinding ring and the top member are moveable relative to one another. The grinding ring and the top member are typically ring-shaped; the terms “grinding ring” and “top ring” may hereinafter be used.
Typically the required relative movement between the grinding elements and the lower grinding ring is achieved by driving the grinding ring, while the top ring is held against rotation. The grinding elements, which are typically steel balls or rollers, are not driven. They may be fixed in position, or free to precess.
The mill with which the invention is concerned is of the type having a rotating port ring generally as described in EP 0507983A. Such a port ring is provided, between the periphery or circumference of the grinding ring and an inclined liner (which may also be called a skirt, or gusset) carried by the wall of the mill. There is provided an annular passage or “throat”, just outboard of the grinding ring. Air flows upwardly through the port ring. The port ring has inner and outer annular walls, between which there are a plurality of spaced-apart, inclined, vanes, separating openings through which air can flow. The port ring rotates with the grinding ring and the vanes impart a desired vector to the generally upwards air flow.
Around its 360 degree extent the port ring may define only openings and the through-thicknesses of the vanes. That is to say there is in effect an annular passage separated into individual openings only by the through-thicknesses of the vanes.
The inner and outer annular walls of the port ring are fixed. The gap between them, in which the vanes are located, cannot be varied.
The size of the gap is selected to provide an optimal air flow rate, which assures efficient advancement of coal fines towards the combustion apparatus. Control of air flow rate is of critical importance in a mill. Too high an air flow rate for a given throughput gives an increased risk that non-combustible materials may be carried forward to the combustion apparatus, along with desired coal fines. Too low an air flow rate, and the coal fines are not all carried to the combustion apparatus, leading to inefficient operation.
The rotating port ring is an excellent and successful mill feature but it is not optimal with coal sources which give rise to incomplete grinding; especially with coal sources which contain inclusions of rock. In such circumstances some unground pieces may be too big to fall through the port ring, and back into the material to be fed to the grinding zone, or scrapped.